1. Field of the Invention
The present invention relates generally to air vents for ventilating a device having a light source. More specifically, the present invention relates to an air vent that blocks direct light emitted from the light source without substantially restricting the flow of air through the vent.
2. Background Information
Devices requiring a light source for generating light, such as a projection display apparatus, an arc lamp, a laser device and the like, need to be ventilated to dissipate heat generated by the light while minimizing or preventing the escape of direct light emitted by the device. These types of devices are therefore typically equipped with an air vent.
The air vent permits the exchange of warm air from the interior of the device for cooler air exterior to the device. Projection display apparatuses in particular are often further equipped with fans to increase the air flow so as to accelerate the exchange of air. Thus it is important to provide air vents that do not restrict or impede the flow of air from the interior to the exterior of the device to allow for maximum ventilation.
The concurrent requirement of minimizing or preventing the escape of direct light from the device works against designing air vents that allow unrestricted air flow. For example, a typical prior art air vent for a projector uses louvers to create the openings that permit the flow of air into and out of the device""s housing. If the louver angle and depth is too shallow then the air flow may be relatively unrestricted, but the vent will allow direct light to escape from the housing. This is especially undesirable for a presentation projector device, since the light will interfere with the darkening of the presentation room required for proper viewing of the projected presentation. Alternatively, if the louver angle is too steep or the depth of the louvers too deep (i.e. if the length of the vanes of the louver are too long), then the air flow is severely restricted. While this has the effect of blocking the escape of at least some of the direct light, it results in an undesirable increase in the amount of heat buildup from the light source or other heat emitting components in the interior of the device housing. FIGS. 1a-1b illustrate an example of the latter type of prior art projector air vent 100. The frontal view shows a series of parallel angled louvers 120 set into the projector housing and intermittently connected by vertical connecting ribs 105. As shown in a sectional view 110 of the prior art projector air vent, the steep angle 115 of the louvers 120 restricts the air flow, thereby impeding the air vent""s ability to dissipate heat emitted by the light source 125. At the same time, some direct light can still escape 130, thus interfering with the proper viewing of the projected presentation.
Another drawback of prior art air vent designs is the reliance on increased fan speed to overcome the air flow restriction of vent designs that attempt to block direct light from escaping. This has the undesirable effect of increasing the noise produced by the device. In the context of a presentation projection device, the noise can interfere with the effective use of the device to deliver a presentation in a conference room setting.
The challenge of designing an air vent that blocks direct light from escaping from the device housing is fairly straightforward; the air vent must be constructed so as to interfere with all direct light paths regardless of the vantage point of the user of the device. On the other hand, the challenge of designing a vent that minimizes the restriction of air flow while blocking direct light requires a complex analysis of the causes of air flow restriction through the vent: flow between parallel plates (the flow along the surfaces of the parallel louvers that comprise the vent), flow contraction at the entrance to the vent, and changing the direction of the flow (the angle of the louvers measured from a perpendicular to the vent). As explained in the following paragraphs, the causes of air flow restriction are discussed in terms of the loss of air flow pressure from the time the flow of air enters the vent until the time the flow of air exits the vent.
The air flow pressure loss between the parallel louvers or substantially parallel louvers is attributed to the boundary layer phenomenon, whereby air particles on the inside walls of the louver surfaces are at zero velocity. An example of the boundary layer phenomenon is illustrated in FIG. 2. The zero velocity air particles 210 create a layer 215 that increases in thickness as the flow of air 220 moves through the parallel louvers 205. Eventually, the outer portion of the boundary layers 215 on the opposing interior surfaces 225 of the parallel louvers 205 contain air particles having a reduced flow air velocity that nearly converge 230 such that air no longer flows efficiently through the air vent 200. The pressure loss between the parallel louvers 205 scales linearly with the length 250 of the vanes of the parallel louvers 205 and inversely with the square of the distance between the louvers 240 (also referred to as the pitch of the vanes). The pressure loss also scales linearly with the velocity of the air flow 245 upon entering the air vent 200. So increasing the speed of a fan to increase the air flow velocity will only result in a proportionate increase in the pressure loss through the air vent 200. Consequently, the length of the vanes of the parallel louvers 250 as well as the distance between adjacent louvers 240 (the pitch) are important factors to consider when designing an air vent that minimizes the loss of air flow pressure.
The pressure loss due to the contraction of the air flow at the entrance to the vent is a function of the open area fraction, according to a classic reference on the subject, Kays, W. M, and London, A. L., Compact Heat Exchangers, 3rd Ed., McGraw-Hill, New York, 1984. An illustration of the entrance 315 to a set of parallel louvers 320 in a typical prior art air vent 310 is shown in FIG. 3. The open area fraction is roughly equal to the thickness of the solid louver material 325 divided by the distance between adjacent louvers 330 (pitch), not taking into account the reduction of the size of the entrance due to the support ribs (not shown). The air flow contraction pressure loss varies as the square of the air flow velocity.
The pressure loss due to the change in direction of the air flow, i.e. the angular deflection of the air flow caused by the angle of the louvers measured from a perpendicular to the air vent, scales as a polynomial function of the angle, according to a classic reference on the subject, Fried, E., and Idelchik, I. E., Flow Resistance: A Design Guide for Engineers, Hemisphere Publishing, New York. An illustration of air flow restriction 115 caused by the angular deflection of the flow is shown in the sectional view 110 of the prior art vent in FIG. 1b. As is illustrated in FIG. 4, a graph entitled Louver Pressure Loss Coefficient 410 shows the coefficient of pressure loss 415 as a function of the louver angle 420. As shown, the coefficient of pressure loss is only 5 when the louver angle is 40 degrees, but quickly increases to a coefficient of pressure loss of 20 when the louver angle is 60 degrees. As expected, the shallower angles will not restrict air flow as much as the steeper angles. However, shallower angles also will not block as much direct light as the steeper angles, an undesirable result.
Accordingly, a new approach is needed for venting devices that takes into account all of the factors that affect air flow pressure loss through the vent as well as the requirement of blocking the emission of direct light from the device from most, if not all, vantage points of the device user. An air vent design that takes into account all of these factors and requirements presents a unique set of challenges, requiring a new and novel solution.
According to one aspect of the invention, an air vent apparatus is provided in which a stacked chevron design is employed to allow increased air flow while blocking the escape of direct light. In one embodiment of the present invention, the stacked chevron is symmetrically disposed in the vent housing. In another embodiment of the present invention, the stacked chevron is asymmetrically disposed in the air vent housing so that a vane of the chevron extending towards the interior of the vent is substantially perpendicular to the air vent housing and substantially parallel to the source of the flow of air. The use of a stacked chevron design allows the device housing be constructed with vanes having a range of shallower angles than those of prior art air vents so as to minimize the restriction of the flow of air through the air vent, while at the same time blocking all or nearly all of the direct light emitted from the device""s light source. Numerous variations in the length of the vanes of the chevron (i.e. the depth of the vent), the vane angle, and the pitch (i.e. the distance between the stacked chevrons) may be employed to achieve a suitably optimal air vent for a number of different devices, including presentation projectors, arc lamps, laser devices and the like.